1. Field of the Invention
The present invention relates to a rear wheel suspension system of a vehicle, and more particularly to a rear wheel suspension system of a vehicle wherein a mounted part at a front side of suspension system at the rear wheels is configured to respond to lateral forces applied to the rear axle during turning of the vehicle to provide a turn characteristic with an under steering, bias to, thereby ensure the stable turning of the vehicle.
2. Description of Prior Art
Generally, a chassis spring in a suspension system of a vehicle is a mechanical element mounted between wheels to absorb shocks transmitted from the roadbed and vibrations of tires of a vehicle to minimize shocks and vibrations transmitted to the vehicle, thereby improving the ride feeling. The chassis spring is generally a leaf spring, a coil spring and an air spring, or the leaf spring is usually adopted due to heavy-duty trucks for its reasonable price, easy maintenance and good durability.
FIG. 1 is a side view for illustrating a mounted state of a leaf spring to rear wheels of a vehicle according to the prior art, wherein, the leaf spring 31 is centrally fixed to a rear axle 32 via U-bolt 33 and is directly coupled to a first mounting bracket 35 fixed to a side frame 34 of a body via a bolt 36, while the leaf spring 31 is coupled at a rear end thereof to a shackle 37, and the other end of the shackle 37 is fixed to a second mounting bracket 35' coupled to the side frame 34 via the bolt 36.
The leaf spring 31 mounted on the side frame 34 repeats vertical elongation and restoration according to shocks transmitted from the roadbed and loads applied to the body while a vehicle is running due to its materialistic kinematics property; and the rear axle 32 fixed to an approximate central part of the leaf spring 31 via the U bolt 33 vertically moves along a circular arc showing a predetermined shape of a trace around a rotating central point (0) existing at a front predetermined position of the body. A point (A), which is a coupling position of the leaf spring 31 for fixing the rear axle 32, maintains a normal position during a normal running state of a vehicle but moves to a point (B) or a point (C) according to load change of the body, or shock change from the roadbed, as illustrated in FIG. 2.
In other words, the point (A) which is a fixing point of rear axle 32 on the leaf spring 31 moves to the point (B) when the body bumps, while the point (A) moves to the point (C) when the body rebounds. As mentioned above, when the point (A) which is the fixing point of rear axle 32 on the leaf spring 31 vertically moves from the point (B) to the point (C) along the trace on the circular arc around the rotating central point (0), the point (A) moves back and forth up to maximum interval of "l" as is seen in FIG. 2.
Accordingly, there is a disadvantage in that wheels at the rear axle 32 are changed in running directions thereof when a vehicle rolls, thereby resulting in an axle steer phenomenon.
Meanwhile, the axle steer phenomenon which occurs according to changes of load applied to the leaf spring 31 of a running vehicle can be severely generated when the vehicle turns around, and FIG. 3 illustrates a suspension system of rear wheels when a vehicle runs straight and turns around.
In other words, as illustrated in FIG. 3, the rear axle 32 supporting rear wheels maintains in parallel with a front axle 40 supporting front wheels when a vehicle runs straight. However the running vehicle turns to the right, a centrifugal force is applied to an opposite direction of a body of the vehicle, such that, load of the body is applied to the leaf spring 31 supporting the rear axle 32 from the opposite direction of turn due to the centrifugal force. The leaf spring 31 positioned in the opposite direction of turn moves from the point (A) which is the fixing point of the rear axle 32 to the point (B), as it is deformed while the body bumps.
At the same time, the leaf spring 31 positioned in the turning direction moves from the point (A) which is the fixing point of the rear axle to the point (C), as it is deformed while the body rebounds.
Successively, one end of rear axle 32 in the opposite direction of turn is pushed backward to a maximum of "l" from an extension line (L) of a normal rear axle 32, while one end of rear axle 32 in the turn direction is pushed forward to a maximum of "l" from the extension line (L) of the normal rear axle 32.
In other words, there is a disadvantage in that, when a vehicle turns, the stance of the vehicle body is slanted to an opposite direction of the turn due to centrifugal force applied to the body, thereby resulting in an axle steer phenomenon where the rear axle 32 and wheels 38 mounted thereto are facing in the opposite direction of turn due to kinematics property of the leaf spring 31.
As mentioned above, when rear axle 32 is changed in direction thereof while a vehicle turns, a so-called over steer phenomenon occurs where an actual turning radius becomes smaller than a steer degree made by steer manipulation of a driver, in addition to the axle steer phenomenon, thereby decreasing the stability of turns.
Furthermore, when the direction of the rear axle 32 is changed to allow rear wheels mounted thereto to face in the opposite direction, wheel alignment of the rear wheel 38 mounted on the rear axle 32 is changed to toe-out, by which friction between tires of the rear wheels 38 and the roadbed is deepened to thereby increase the wear of the tires.